The present invention relates to a thermally stabilized polyorganosiloxane oil and in particular to a fusing system for fusing toner images in an electrostatographic printing apparatus which employs such an oil as a release agent.
In a typical electrostatographic reproducing apparatus, a light image of an original to be copied is recorded in the form an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles which are commonly referred to as toner. The visible toner image is then in a loose powdered form and can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be photosensitive member itself or other support sheet such as plain paper.
The use of thermal energy for fixing toner images onto a support member is well known. In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
Typically, thermoplastic resin particles are fused to the substrate by heating to a temperature of between about 90.degree. C. to about 160.degree. C. or higher depending upon the softening range of the particular resin used in the toner. It is not desirable, however, to raise the temperature of the substrate substantially higher than about 200.degree. C. because of the tendency of the substrate to discolor at such elevated temperatures particularly when the substrate is paper.
Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means: a roll pair maintained in pressure contact; a belt member in pressure contact with a roll; and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines or process conditions.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate or belt members. The concurrent transfer of heat and the application of pressure in the nip effects the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus, increasing the background or interfering with the material being copied there. The so called "hot offset" occurs when the temperature of the toner is raised to a point where the toner particles liquefy and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member. The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser roll, and accordingly it is desired to provide a fusing surface which has a low surface energy to provide the necessary release. To ensure and maintain good release properties of the fuser roll, it has become customary to apply release agents to the fuser members to ensure that the toner is completely released from the fuser roll during the fusing operation. Typically, these materials are applied as thin films of, for example, silicone oils to prevent toner offset.
The silicone oils employed as release agents in these practices are typically polyorganosiloxanes, more specifically, polydimethylsiloxanes, which are used in a range of viscosities of from about 100 centistokes to 13,000 centistokes. While capable of performing satisfactorily these silicone oil release agents suffer from certain deficiencies. In particular, they tend to show an increase in viscosity and eventually gel when held at elevated temperatures with the consequence that the release agent management delivery system can be adversely affected. For example, the oil can gel while on the fuser roll or in the supply lines of the release agent management system. As previously discussed the typical fusing systems in electrostatographic printing apparatus have a heated fuser roll heated to temperatures of the order of 90.degree. to 160.degree. Centigrade and sometimes to temperatures approaching 200.degree. Centigrade. An additional problem associated with these silicone oils at elevated temperatures is the generation of silicone oil vapor which is a detrimental by-product in that it tends to form insulating layers on the electrical circuits and contacts and may therefore interfere with the proper functioning of these circuits and contacts. Furthermore, depending on the chemical makeup of the silicone oils the vapors released at elevated temperatures may include environmentally undesirable materials such as benzene, formaldehyde, trifluoropropionaldehyde.